AJAX and PHP

Chapter 4
}
}
?>
return ($value == 'true' || $value == 'on') ? 1 : 0;
13. Test your script by loading http://localhost/ajax/validate/index.php in a
web browser.
What Just Happened?
The AJAX validation technique allows us to validate form fields and at the same time inform
users if there were any validation errors. But the cherry on the top of the cake is that we are doing
all of this without interrupting the user's activity! This is called unobtrusive form validation.
The unobtrusive validation is combined with a pure server-side PHP validation that happens when
submitting the form. At the server, both validation types are supported by a PHP script called
validate.php, with the help of another PHP script called validate.class.php.
Let us examine the code, starting with the script that handles client-side validation, index.php. In
this validation example, the client page is not a simple HTML file, but a PHP file instead, so portions
of it will be still dynamically generated at the server side. This is necessary because we want to
retain the form field values when the form is submitted and server-side validation fails. Without
the help of the PHP code, when the index page is reloaded, all its fields would become empty.
index.php starts with loading a helper script named index_top.php, which starts the session by
calling session_start(), defines some variables and a function that will be used later in index.php,
and initializes some session variables ($_SESSION['values'] and $_SESSION['errors']) that we
will be using to avoid PHP sending notices about variables that are not initialized.
Notice the onload event of the body tag in index.php. It calls the setFocus() function defined in
validate.js, which sets the input cursor on the first form field.
Later in index.php, you will see the following sequence of code repeating itself, with only
small changes:
Desired username: